Improve board interactions with a custom gesture detector

[veloce]

So far we've been using a GestureDetector from flutter. In this PR, it is replaced by the low level Listener which allow to listen to raw events and make a custom gesture detector tailored to lichess usage.

This brings many changes in the way to interact with the board:

Playing:

• pieces can now be moved with 2 tap down events, instead of a a full tap (pointer down followed by another pointer up event): this allows for a faster interaction
• premoves are not anymore cleared when selecting another piece: this matches lichess website behaviour and allow to prepare another move along with the premove that is currently set

Drawing shapes (experimental)

• drawing shapes is now possible while keeping the normal board play interaction (before it was either one or another mode)
• one can draw a shape by holding a finger to an empty square while using a second finger to draw a shape anywhere in the board
• a double tap on an empty square will clear all shapes at once
• to clear a single shape is still supported: draw the same shape again